Funky things you can do with the call stack, or general runtime funiness:

<ul>
<li>reverse debugging
    <ul>
    <a href="http://www.gnu.org/software/gdb/news/reversible.html">[1a]</a>
    <a href="http://sourceware.org/gdb/wiki/ReverseDebug">[1b]</a>
    <a href="http://en.wikipedia.org/wiki/TotalView">[2]</a>
    </ul>
<li><a href="http://en.wikipedia.org/wiki/Continuation">continuations</a>, which can <A href="http://en.wikipedia.org/wiki/Continuation#Disadvantages">get confusing</a>
<li><a href="http://en.wikipedia.org/wiki/Interactive_programming">interactive programming</a> / <a href="http://en.wikipedia.org/wiki/Live_coding">live coding</a>
</ul>